1. Field of the Invention
The present invention relates to an axle driving apparatus equipped with a hydrostatic transmission, and more particularly to a charge pump for feeding pressurized oil into a closed circuit of the hydrostatic transmission. The charge pump is reduced in weight and has improved durability.
2. Related Art
Conventionally, it is a well-known technique for a charge pump to be integrally attached to a hydrostatic transmission (hereinafter referred to as an HST) in order to feed pressurized oil thereto. Said technique has been proposed by this applicant in U.S. Pat. No. 5,031,403, in which the charge pump is bolted onto the lower horizontal surface of the center section of the HST.
When operating oil, circulating between a hydraulic pump and a hydraulic motor, both mounted onto the center section, is fed by the charge pump, the incorporation of a charge relief valve into the center section is required in order to maintain the proper feed oil pressure. Therefore, this applicant, by eliminating this exclusive charge relief valve, has further improved the above-mentioned technique, as proposed in Japanese Utility Model Publication No. Hei-6-12276.
In the above-mentioned invention, the casing of the charge pump is not fixed to the lower horizontal surface of the center section, but is pressed to the lower surface thereof so that the discharge port of the charge pump is closely connected with the supply port of the center section. When the pressure of the oil discharged from the charge pump exceeds the biasing force of a spring, the entire charge pump leaves the lower horizontal surface of the center section, the discharge pump temporarily disconnects from the supply port of the charge pump, and the oil pressure discharged from the discharge pump is adjusted corresponding to the biasing force of the spring. In effect, the charge pump itself functions as the charge relief valve.
The charge pump comprises three parts: an inner rotor, an outer rotor and a pump casing. They are wrought of metal, such as iron, or molded of metal from the iron family such as sintered metal, superior in productivity and generally larger in weight, causing damping to decrease when the charge pump is under relief operation, and resulting in instable relief performance. When the center section, made of aluminum, is struck at the lower horizontal surface by the charge pump made of iron, there is concern that the lower surface will wear after long use.
When the HST is used for driving axles of the vehicle and the vehicle is parked on a slope with the engine stopped, the charge pump is not driven and the hydraulic motor is subjected to a load, causing the wheels to rotate, and pumping the operating oil circulating in the closed circuit of the center section. Thus, in the state where the operating oil in the closed circuit is under pressure, with the lapse of time, the oil leaks from a sliding portion of each piston in a cylinder block or a slidably rotating portion thereof. When negative pressure develops in the closed circuit, small quantities of air in the operating oil expand to naturally rotate the hydraulic motor, allowing the vehicle to roll away along the slope.